1. Field of the Invention
The present invention relates to a printer that allows dual-way printing and forms dots on a printing medium by main scans in both a forward direction and a backward direction, thereby printing an image. More specifically the present invention pertains to a technique that prints a test pattern in such a printer.
2. Description of the Prior Art
An ink jet printer is one typical example of printing apparatuses, in which a print head reciprocates in a main scanning direction to scan a printing medium in a sub-scanning direction and print an image. In these printers, the print head generally has a plurality of nozzles (hereinafter referred to as the multi-head), in order to improve the printing speed. In printers that allow color printing, the multi-head is generally provided for each color ink.
Some of these printers create dots not only in the course of the forward motion of the main scan but in the course of the backward motion of the main scan, in order to further improve the printing speed. In this printer, deviation of the dots created in the course of the backward motion of the main scan from the planned positions corresponding to the dots created in the course of the forward motion of the main scan results in unsuccessful printing of an image. This phenomenon is caused by a variety of factors, for example, the backlash or the play required for the driving mechanism of the printer and the difference in thickness of the sheet used as the printing medium.
FIGS. 42 and 43 show a deviation of dots due to the thickness of the sheet. In the example shown in FIG. 44, a dot dt11 is formed on a sheet of paper
in the course of the forward motion of the main scan, and a dot dt12 is formed in the course of the backward motion of the main scan to be adjacent to the dot dt11. Nozzles Nz spray droplets of ink Ik11 and Ik12 at positions shown in FIG. 44 by taking into account the speeds of the main scan in the forward direction and in the backward direction. The ink droplets Ik11 and Ik12 draw the loci shown in FIG. 44 and reach the target positions to form the dots dt11 and dt12. PA1 (a) creating dots at a first timing that forms a first pattern in the course of the main scan in the forward direction, wherein the first pattern includes a dark portion having a certain area and a light portion having an area greater than the area of the dark portion that alternately appear at a first cycle in the main scanning direction in a predetermined first section of the printing medium; and PA1 (b) creating dots at a second timing that is supposed to form a second pattern in the course of the main scan in the backward direction, wherein the second pattern includes a dark portion having a certain area and a light portion having an area greater than the area of the dark portion that alternately appear at a second cycle in the main scanning direction in a predetermined second section of the printing medium, the predetermined second section at least partly overlapping the predetermined first section, and wherein the dark portion of the first pattern and the dark portion of the second pattern appear at a fixed interval in the main scanning direction in the overlapped area.
FIG. 45 shows formation of the dots when a sheet of paper PA2 has a greater thickness. In this case, the distance between the nozzle Nz and the sheet of paper PA2 is smaller than the distance between the nozzle Nz and the sheet of paper PA1 shown in FIG. 44. When ink is sprayed at the same timings as those in FIG. 44 in the forward course and the backward course of the main scan, droplets of ink Ik21 and Ik22 draw the loci shown in FIG. 45 and reach the illustrated positions to form dots dt21 and dt22. The resulting dots dt21 and dt22 do not adjoin to each other, so that the resulting image is different from the image to be printed originally. In order to obtain the image to be printed originally, the timing of spraying the ink in the backward course of the main scan should be delayed from the timing shown in FIG. 45.
The technique of adjusting the print timing based on a test pattern is adopted to eliminate such a deviation. This technique prints a predetermined test pattern while varying the dot print timing in the forward course and the backward course of the main scan. The dot print timing is selected that gives the optimum printing result among the test patterns printed at various timings. As discussed above, the thickness of the sheet is one factor that causes the deviation of the print timing. The adjustment of the print timing should thus be carried out by the user of the printer, in addition to the time of the delivery of the printer.
A line pattern as shown in FIG. 46 is conventionally used as the test pattern. The upper half of each line shown in FIG. 46 is printed by the forward motion of the main scan, whereas the lower half is printed by the backward motion of the main scan. Varying the dot print timing causes a change of the positional relationship between the upper half and the lower half of each line as shown in (a) through (e). The line pattern of (c) is a favorable image in which there is no relative deviation of the lower half from the upper half. The timing corresponding to the line pattern of (c) should accordingly be selected as the dot print timing.
Another proposed technique (JAPANESE PATENT LAYING-OPEN GAZETTE No. 7-81190) fills a predetermined area with dots to form a solid test pattern when the dot print timing is appropriate. In case that the dot print timing is deviated from the appropriate timing, white streaks where no dots are formed appear in the area that is supposed to be solid. This technique selects the dot print timing that does not cause such white streaks as the appropriate dot print timing.
The print timing may, however, not be adjusted adequately with the line test pattern as shown in FIG. 46. FIG. 46 shows the enlarged test patterns for convenience of explanation. In the actual state, however, each line consists of one array of dots, so that it is difficult to distinguish the state of (b) or (d) from the ideal state of (c). The distinction is especially difficult for the unskilled user of the printer who is unfamiliar with the test patterns. In the recent advanced printers of the high resolution, unsuccessful adjustment of the print timing may result in unsuccessful printing of an image.
The print timing may also not be adjusted adequately with the solid test pattern where a predetermined area is filled with dots. The white streaks are extremely narrow, so that the ink blot on the paper makes it difficult to identify such white streaks.